A heterojunction field effect transistor takes an advantage of two-dimensional electron gas generated along the interface of the heterojunction for its operation. One known example is a high electron mobility transistor (HEMT).
One example of an HEMT is shown in FIG. 1. The HEMT shown in FIG. 1 is the one disclosed in U.S. Pat. No. 4,827,320. In FIG. 1, on the top surface of a substrate 2 of semi-insulating material, for example, gallium arsenide, GaAs, a channel layer 4 of indium gallium arsenide, In.sub.y Ga.sub.1-y As, is disposed, where y is a value of from 0.1 to 0.25. On the channel layer 4, an electron supply layer 6 of aluminum gallium arsenide doped with silicon, Si, as an impurity, i.e. n.sup.+ Al.sub.x Ga.sub.1-x As (x being a value between 0.05 and 0.20), is disposed. A cap layer 8 of n.sup.+ GaAs is disposed on the electron supply layer 6. On the top surface of the cap layer 8, a source electrode 10 and a drain electrode 12 are disposed spaced from each other. Broken lines extending downward from the source and drain electrodes 10 and 12 in FIG. 1 indicate that the metals constituting the electrodes 10 and 12 diffuse so that the electrodes 10 and 12 contact the channel layer 4. A portion of each of the cap layer 8 and the electron supply layer 6 between the source and drain electrodes 10 and 12 is removed, and a gate electrode 14 is disposed there on electron supply layer 6.
In the above-described HEMT, the electron supply layer 6 is depleted due to a band-gap difference .DELTA.Ec of, for example, about 0.3 eV, between the electron supply layer 6 and the channel layer 4, so that the impurity, Si, in the electron supply layer 6 is ionized. Electrons generated in the electron supply layer 6 by this ionization are stored in the channel layer 4 along the interface with the electron supply layer 6 due to the difference in electron affinities. The stored electrons produce a two-dimensional electron gas channel. In this case, a peak value of the electron concentration in the channel layer 4 is, for example, from 1.times.10.sup.12 cm.sup.-2 to 2.times.10.sup.12 cm.sup.-2.
When a voltage is applied between the source electrode 10 and the drain electrode 12, the electrons in the two-dimensional electron gas move between the two electrodes 10 and 12 at a high velocity because they are scattered little by dopant impurity. The field effect transistor operation of this HEMT is provided by controlling electrons in the two-dimensional electron gas channel by means of the electric field produced by the gate electrode 14.
In HEMT's as described above, the composition of the channel layer 4 is In.sub.y Ga.sub.1-y As, where y is a value between 0.1 and 0.25, and is different from that of the GaAs layer 2. Accordingly, the channel layer 4 and the GaAs layer 2 have different lattice constants, which causes lattice mismatching between the two layers. This, in turn, causes crystalline defects in the channel layer 4, so that the two-dimensional electron gas mobility decreases, and the performance of the HEMT is degraded. Further, because of the composition of the channel layer 4 being as stated above, the bandgap difference between the channel layer 4 and the electron supply layer 6 cannot be made larger than about 0.3 eV as stated above and, accordingly, the concentration of two-dimensional electrons in the channel layer 4 cannot be increased.
Therefore, an object of the present invention is to provide a heterojunction field effect transistor with little lattice mismatching.
Another object of the present invention is to provide a heterojunction field effect transistor having an increased difference in bandgap between a channel layer and an electron supply layer.